In current wireless infrastructure systems, most RF power amplifiers are designed for operation in a specific narrow band of frequency and for a particular air interface technology such as CDMA, GSM, WCDMA, WiMax, LTE, or others. With the current plans by wireless network operators to migrate to LTE service by initially overlaying this service with existing 2G or 3G service, there is an incentive for operators to deploy equipment that can be reconfigured to operate across multiple bands or to be reconfigured for various air interface waveforms in order to avoid complete replacement of installed equipment.
A common issue in implementation of a multi-band or multi-mode power amplifier is in achieving optimum performance from the transmitter power amplifier over a broad frequency range or for multiple waveforms. The gain, linearity, and power added efficiency (PAE) performance of a power amplifier (PA) is heavily dependent on the complex load impedance presented to the transistors within the PA. Based on the characteristics of the transistors, a specific complex load impedance or narrow range of load impedance values will provide the optimum PAE. Often the optimum output power is achieved at a specific complex load impedance that is at a different impedance value than that required to achieve optimum PAE. Also, the optimum linearity performance in terms of error vector magnitude (EVM), adjacent channel power ratio (ACPR), or two tone intermodulation ratio (TTIR) is achieved at another possibly different load impedance than that required to achieve optimum PAE or output power. Since the final power amplifier in a radio transmitter is the dominant factor in the overall power consumption, efficiency, and linearity of the transmitter, it is normally critical to transform the actual impedance of the load through a matching network to present the ideal load impedance to the power amplifier transistors depending on which performance parameter must be optimized.